Chimpanzees and people already share much when it comes to our evolutionary history, and the latest research shows we have a similar appreciation for fairness as well.

A study published in the Proceedings of the National Academy of Sciences reveals the first evidence for the trait, which scientists had previously thought was exclusive to higher-thinking Homo sapiens, in chimps. The common ancestral lineage of humans and chimps also means that the discovery could provide insight into how human behavior evolved.

Researchers at Emory University’s Yerkes National Primate Research Center and Georgia State University played “the ultimatum game” with chimpanzees to find out if chimps are sensitive to how rewards are divided up after collaborating to obtain them. Chimpanzees, like people, acted more equitably than selfishly when they had to work together to obtain food rewards.

That’s not a complete surprise, given what is already known about chimpanzee social structures. They are social and cooperative animals that work together to hunt, defend their territory and share food. Some studies suggest chimps even keep track of how frequently other chimpanzees support or help them. But while some past studies have suggested that chimpanzees might split shared bounty equitably, none have demonstrated a clear sense of fairness until now.

“We were surprised people hadn’t been able to show it experimentally before given all that anecdotal information that we have, from wild chimpanzees to chimps in captivity,” says Darby Proctor, the study’s first author.

In the study, researchers created a modified version of the ultimatum game, a classic test for studying fairness. When the experiment is performed with people, one participant is usually given money that he or she can divide up in any way with a second anonymous participant in another room. If the second participant accepts the offer, both go home happy, but if he or she rejects the offer, neither participant keeps the cash. It’s in the best interest of the first participant to play fair — if the participant acts too selfishly, he or she might not get anything at all.

Instead of money, the researchers used food to incentivize the chimps. But because chimpanzees have a difficult time making careful decisions when food is right before them, the scientists trained the animals on the symbolic meaning of tokens that represented different food scenarios. One token represented an equitable offer (the chimps get the same number of banana slices), while the other represented a selfish offer (the first chimp gets five banana slices, the second gets only one). The first chimpanzee would select a token to hand to its partner chimp, who could either redeem the reward by giving the token to the experimenter, or do nothing and refuse the offer. The tokens also made the game more like the human version: like money, the tokens are abstract items that have to be exchanged to realize their value.

In order to determine whether fairness was potentially driving the chimpanzees’ behavior, the scientists ran two versions in the game. In the first, the chimpanzees were trained to understand that their choice of tokens was strictly straightforward: If they chose the fair offer, each chimp would receive the same number of banana slices, and if they chose the unfair offer, the chooser would receive five banana slices and the partner would receive one. In this round, the chimps were almost uniformly selfish, choosing the unfair offer close to 90 percent of the time.

But when the ultimatum portion of the game began, and the number of banana slices depended on whether the receiving chimp cooperated, the animals chose the equitable offer around 70 percent of the time.

“The chimpanzees were clearly paying attention to what their partners’ outcomes were and adjusting their behavior depending on whether or not their partner could affect the outcome,” says Sarah Brosnan, the study’s senior author. “If their partner couldn’t do anything, they went ahead and took the option that gave them the most rewards. But if their partner had the potential to change the outcome, then they actually switched their behavior.”

The researchers then played the game with a group of 20 children between the ages of 2 and 7 who played for stickers — at those ages, children have only a limited appreciation of fairness, and likely still interpret fair play mostly in terms of immediate benefits or rewards, as the chimps did. So it wasn’t surprising that the children showed similar responses to the game that the chimpanzees did: exhibiting selfish behavior when the rewards were not dependent on sharing, and showing more generosity and a willingness to give up some stickers when the result depended on co-opting their partner to cooperate.

Proctor says a sense of fairness likely evolved alongside the ability to cooperate. Working together may have helped evolutionary ancestors achieve the most beneficial diet, and identifying when they were getting a fair deal possibly helped them recognize when such cooperation was worth the effort.

“It really opens the door to figuring out where in our evolutionary history and what sort of adaptive pressures led to this human sense of fairness,” Proctor says. “Now that we’ve seen it in one of our closest living relatives, it opens the door for us to look for it in other species as well.”